cmStandardLevelResolver: Avoid unnecessary flags, fix unset level logic

The changes are part of CMP0128.

When the standard level is unset:
* Flags are added if extension mode doesn't match the compiler's default.
  Previously logic only worked if LANG_EXTENSIONS was ON. Fixes #22224.
* The full flag is used. Previously CMAKE_LANG_EXTENSION_COMPILE_OPTION was
  used. This was only supported for IAR.

Otherwise:
* Avoid adding flags if not necessary per the detected compiler defaults.
* Fixed check for when the requested standard is older. It now matches the
  nearby comments.

I reworded the fallback comment as its logic was a bit difficult to wrap my
head around.

1 files changed, 10 insertions, 0 deletions

diff --git a/Help/release/dev/compile-features-standard-logic-rework.rst b/Help/release/dev/compile-features-standard-logic-rework.rst
new file mode 100644
index 0000000..432a756
--- /dev/null
+++ b/Help/release/dev/compile-features-standard-logic-rework.rst
@@ -0,0 +1,10 @@
+compile-features-standard-logic-rework
+--------------------------------------
+
+* The :manual:`Compile Features <cmake-compile-features(7)>` functionality now
+  correctly disables or enables compiler extensions when no standard level is
+  specified and avoids unnecessarily adding language standard flags if the
+  requested settings match the compiler's defaults. See :policy:`CMP0128`.
+
+* :prop_tgt:`<LANG>_EXTENSIONS` is initialized to
+  :variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT`. See :policy:`CMP0128`.